Graphene has been defined as a monolayer-thick sheet of sp2-bonded carbon atoms that form a honeycomb hexagonal crystal lattice. Strictly speaking, graphene is as defined above; however, in this specification, a carbon film consisting of the plurality of sheets up to and including 100 sheets stacked is also referred to as graphene.
Graphene is made by a variety of methods; the Hummers method is a simple method, and many researches have been conducted on the Hummers method (see Patent Document 1 and Patent Document 2). In the Hummers method, graphite is oxidized with an oxidizer, first. Potassium permanganate is used as the oxidizer, together with which, acid such as sulfuric acid is generally added to promote oxidation effect.
The oxidized graphite (graphite oxide) retains the layered structure, in which the interlayer distance is, however, larger than that of graphite; thus, the layered structure can be easily broken by ultrasonic treatment or the like, whereby oxidized graphene (graphene oxide) can be obtained. The resulting graphene oxide may have at least a sheet of carbon atoms.
The graphene oxide is deposited on an appropriate object surface to be thin in a film shape and reduced, whereby an extremely thin carbon film (graphene) can be formed.